1. Field of the Invention
The present invention relates to an image forming apparatus and a detection method in the image forming apparatus, and in particular, to an image forming apparatus detecting an edge portion of a sheet being fed inside the image forming apparatus and a detection method thereof.
2. Description of the Related Art
In an image forming apparatus, it is necessary to accurately detect an edge portion of a sheet being fed in order to set a position for starting writing of image data on the sheet to a defined position.
Conventionally, a technique of detecting an edge portion of a sheet being fed utilizing an actuator as shown in FIG. 18 has been proposed. Further, optical detection means (an optical sensor) utilizing differences in reflectance and transmittance of light depending on presence or absence of a sheet as shown in FIGS. 19A and 19B has also been used. Furthermore, Japanese Laid-Open Patent Publication No. 2003-223088 discloses a technique of providing positional displacement detection means in a direction perpendicular to a feeding direction, and correcting a position for starting writing of an image according to the calculated amount of displacement in a main scanning direction. Japanese Laid-Open Patent Publication No. 2003-248410 discloses a technique of detecting a leading edge of a sheet by a reading mechanism having reading elements arranged in a feeding direction, and determining time to start image formation. Japanese Laid-Open Patent Publication No. 2005-012783 discloses a technique of detecting positional displacement of a sheet according to a change in the amount of light  utilizing a light receiving element having a light receiving surface with a prescribed length in a feeding direction and a light emitting element, in which either one of the elements is configured to be movable in the feeding direction and thus can detect positional displacements of sheets in plural sizes.
However, none of these techniques can detect a sheet edge unless a trailing edge of a preceding sheet and a leading edge of a subsequent sheet are apart from each other for a prescribed distance, that is, unless there is a sheet interval, during feeding. Therefore, when the sheet interval is less than the prescribed distance or when a preceding sheet overlaps with a subsequent sheet, there occurs a problem that the sheet edge cannot be accurately detected even by using these techniques.